1. Technical Field
The present invention generally relates to improved molds for the production of ophthalmic devices such as contact lenses, intraocular lenses, and other ophthalmic products.
2. Description of the Related Art
Cast molding of ophthalmic devices such as contact lenses is well known. Typically, the contact lens-forming monomer mixture will contain monomers capable of forming suitable polymers, crosslinkers, catalysts, polymerization initiators, and the like which are mixed neat or mixed in suitable diluents or solvents and placed into an anterior mold half. The posterior mold half is then pressed into the anterior mold half and the monomer mixture is polymerized. The lenses obtained from this process conform to the shape of the cavity formed between the two mold halves and exhibit surface characteristics which correspond to the mold surfaces. After the lenses are cast, they are ready for further processing such as cleaning, polishing and/or edging and hydration, as necessary.
The mold pieces used in casting contact lenses have been made of plastic materials which are substantially inert to the monomers employed and to the polymerization process employed. Typically, these plastic mold pieces are discarded after a single use.
Certain contact lenses manufactured by the cast molding process, particularly those known as hydrogels, can have a noticeable frequency of cosmetic defects on their surfaces. Generally, cosmetic defects are sites or areas found on the surface of the lens which can scatter light and indicate the occurrence of an irregular surface area as compared to the rest of the lens surface. While these cosmetic defects cannot usually be seen by the naked eye, they do appear when examined under a slit lamp or by magnification. Although these cosmetic defects are generally not associated with any medical or health concerns, they can lead to slight deficiencies in optimal visual performance of the lenses. In addition, these cosmetic defects can be associated with processing, shipping and handling problems as lenses which contain these defects tend to more easily stick to themselves and to the packaging materials. Finally, improving the polymerization at the lens surfaces results in improved yields of acceptable lenses obtained from the cast molding process.
It has long been known that the presence of oxygen inhibits complete free radical polymerization. Accordingly, conventional cast molding processes have been conducted in inert environments in order to eliminate the effects of oxygen on the polymerization process. These cast molding processes have been found adequate for overall polymerization resulting in a degree of polymerization of greater than about 99% of the bulk lens. Such cast molding techniques have been widely used commercially.
However, oxygen may still have an effect on the free radical polymerization of the contact lens material even when the polymerization is conducted under inert atmospheric conditions. It has been found that certain plastic mold pieces contain sufficient oxygen within the structural matrix of the plastic to adversely affect polymerization at the interface between the mold surface and the surface of the lens. It is believed that the oxygen migrates to the surface of the plastic mold piece during free radical polymerization and inhibits complete polymerization at the lens surface. In addition, the presence of oxygen in the environment is believed to cause reduced crosslinking density at the lens surface. It is this incomplete polymerization or reduced crosslinking density at the lens surface which is believed to cause the cosmetic defects described above.
In order to overcome these problems, molds for making soft contact lenses have been treated to affect their surface properties. For example, U.S. Pat. No. 4,159,292 discloses the use of silicone wax, stearic acid, and mineral oil as additives for plastic mold compositions to improve the release of the contact lens from the plastic molds. U.S. Pat. No. 5,690,865 discloses an internal mold release agent such as waxes, soaps, and oils, including a polyethylene wax having a molecular weight of 5,000 to 200,000 or a silicone polymer having a molecular weight of 2,000 to 100,000. U.S. Pat. No. 5,639,510 discloses a surface-applied surfactant in the form of a uniform layer or very thin film or coating to assist in the release from each other of mold components of a multi-part mold employed in the molding of hydrophilic contact lenses. Polymeric surfactants that can be used include polyoxyethylene sorbitan mono-oleates which are applied to a non-optical surface of the mold, but do not cover the optical surface of the mold.
European Patent Application EP 0 362 137 A1 discloses the coating of molds with a co-reactive hydrophilic polymer such as polyvinyl alcohol (PVA), ethoxylated PVA, or hydroxyethyl cellulose, in order to provide a permanent hydrophilic coating on the lens. The mold coating copolymerizes with the lens material in the mold. Similarly, U.S. Pat. No. 3,916,033 (“the '033 patent”), discloses coating the surface of a mold with polyvinylpyrrolidone to form a coating that is later to come into contact with a previously crosslinked silicone lens. The '033 patent further discloses spreading a coating solution over the mold while held in a chuck, thereby achieving a fairly uniform coating of several thousandths of an inch, after which the wet film is allowed to dry to form a hard glassy polymer layer of about 1 to 5 thousandths of an inch. Finally, monomeric N-vinyl pyrrolidone is dissolved in the coating ready for contact with the silicone lens.
Another example is U.S. Pat. No. 5,779,943 which discloses coating a mold with a hydrophobic latent-hydrophilic material, after which a lens material is molded therein. During curing, the mold coating is apparently transferred to the lens surface. The lens is then treated to convert the coating to a hydrophilic form.
Accordingly, there is a continued need to provide improved molds for manufacturing ophthalmic devices such as contact lenses and other ophthalmic articles placed in or on the eye.